Ammonia synthesis process and apparatus for use therein

ABSTRACT

A method for synthesis of ammonia includes compressing a synthesis gas containing hydrogen and nitrogen in a multi-stage ( 50, 56  and  57 ) centrifugal compressor. The synthesis gas is compressed to a pressure of from about 800 to 900 psia in the first stage ( 50 ) of the compressor, and withdrawn therefrom and subjected to cooling and dehydration by contact with liquid ammonia in a dehydrator ( 54 ). The cooled, dehydrated synthesis gas is then returned to the compressor and introduced into the second stage ( 56 ) thereof. Because of this interstage cooling and dehydration, compressor speed may be reduced and significant savings in power consumption are attained because of the favorable effect of the dehydrator ( 54 ) on the last two stages ( 56, 57 ) of the compressor. Additional power saving is realized because refrigeration requirements for the synthesis loop are reduced. Apparatus to carry out the process includes a centrifugal compressor fitted with a synthesis gas outlet ( 2, 4  and  6 ) connecting the discharge of the first stage ( 50 ) of the compressor with the synthesis gas inlet of the dehydrator ( 54 ), and a synthesis gas intermediate inlet ( 8 ) connecting the inlet of the second stage ( 56 ) of the compressor in flow communication with the synthesis gas outlet of the dehydrator ( 54 ).

BACKGROUND OF THE INVENTION

[0001] The present invention relates to method and apparatus for theproduction of a product by catalytic reaction of a pressurized synthesisgas. For example, one embodiment of the present invention relates to theproduction of ammonia by catalytic reaction of pressurized synthesis gascomprising hydrogen and nitrogen. More specifically, the presentinvention relates to an improved method for purification of make-upsynthesis gas, i.e., synthesis gas which is added to the catalyticreactor to replace reacted synthesis gas.

RELATED ART

[0002] U.S. Pat. No. 3,350,170 issued Oct. 31, 1967 to J. A. Finneran etal, discloses a process for carrying out cyclic synthesis reactions atelevated pressures and is particularly concerned with improvements inthe method of compressing the fresh and recycle synthesis gases in suchprocess. This patent well illustrates the type of synthesis process withwhich the present invention is concerned. As shown in FIG. 1 of U.S.Pat. No. 3,350,170, fresh synthesis gas 10 is introduced into acentrifugal compressor together with gas 42 recycled from a converter 38in which hydrogen and nitrogen are catalytically converted to ammonia.The recycle gas exiting from converter 38 thus contains product ammoniaas well as unreacted hydrogen and nitrogen. The recycle gas isreintroduced via line 24 into the compressor. The compressed outlet gas26 thus comprises a mixture of the recycle gas plus the fresh (make-up)gas introduced via line 10. The product ammonia is separated inseparation vessel 31 and the ammonia-depleted compressed synthesis gastravels to the converter 38 via lines 33, 34 and 35. Line 46 is used toseparate a purged gas from the synthesis loop in order to preventbuild-up of impurities in the synthesis loop defined by lines 42, 24, 26and 33.

[0003] In conventional ammonia synthesis processes, removal of H₂O frommake-up synthesis gas is accomplished by mixing make-up gas containingabout 160 ppm H₂O with recycle gas at the compressor recycle wheelinlet. The gas discharged from the compressor is then cooled and chilledwith H₂O being absorbed in the condensing NH₃. The NH₃ and absorbed H₂Oare separated from the gas in a separator. The converter is fed with gasfrom the separator, which separated gas is substantially H₂O-free or atleast has only a very small residual H₂O content. The separated gas maycontain, for example, about 1.9% NH₃. There are several disadvantageswith this system. The refrigeration power required is higher because ofthe dilution of converter effluent with make-up gas that lowers the NH₃concentration and the dewpoint. This transfers load from the higher tothe lower temperature chillers, which require more power per ton ofrefrigeration. Also, product NH₃ is compressed in the recycle wheel,adding to the power demand imposed on the compressor. A significantimprovement in reduced energy requirements can be realized for thissystem, as shown in U.S. Pat. No. 1,815,243, by incorporating adehydrator.

[0004] A 1989 paper by H. Bendix and L. Lenz of VEB AgrochemiePiesteritz, the former German Democratic Republic (East Germany), waspresented at a meeting of the American Institute of Chemical Engineers.The paper is entitled Results and Experiences on Revamping ofLarge-Scale Ammonia Single-Line Plants and discloses the addition, via aVenturi tube, of liquid ammonia to the synthesis gas discharged from thethird stage of the synthesis gas compressor. The stated purpose is todry the synthesis gas.

[0005] A paper by M. Badano and F. Zardi was presented at the Feb.28-Mar. 2, 1999 Nitrogen '99 meeting in Caracas, Venezuela sponsored byBritish Sulphur Publishing. The paper is entitled Casale GroupExperience in Revamping Ammonia, Methanol and Urea Complexes anddiscloses scrubbing with liquid ammonia, ammonia synthesis gas betweenthe second and third stages of the synthesis gas compressor.

[0006] Another prior art expedient is shown in U.S. Pat. No. 1,830,167and Canadian Patent 257,043. This method involves scrubbing the combinedmake-up and recycle gas stream with liquid NH₃ prior to preheating thestream and sending it to the converter. Normally, there is no need toscrub the recycle stream since there are no impurities in it. A drawbackof the scheme of these patents is that it distributes impurities throughthe entire gas stream. It is then more difficult to effect completeimpurity removal because the impurities are diluted by being dispersedthroughout the entire gas stream. In order to treat the combined stream,the scrubbing apparatus must be much larger and more costly than wouldbe required for scrubbing the makeup gas stream alone, since it istreating a gas volumetric flow which is 4-5 times greater than themake-up gas stream alone. Accordingly, the scheme of U.S. Pat. No.1,815,243 and Canadian Patent 257,043 adds to the scrubbing load bycombining the recycle and make-up streams prior to scrubbing.

[0007] Other prior art expedients include the use of molecular sieves toremove H₂O from make-up gas by adsorption. The concept of dehydratingmake-up gas permits the stream with the highest NH₃ content, theeffluent from the converter, to feed the chilling system. This savesconsiderable refrigeration power and can allow a significant capacityincrease in plants that are limited by the size of the refrigerationcompressor. The power savings is accomplished because of the elevateddew point that results in some condensation with cooling water and atransfer of load from the low to the high temperature chillers whichneed less power per ton of refrigeration. Removal of H₂O by molecularsieves also enables omitting the purge gas chiller that uses the coldestNH₃ refrigeration.

[0008] The H₂O-free (and NH₃-free) make-up gas is then mixed withrecycle gas, compressed in the recycle wheel and fed to the converter.This system has one advantage over competing technologies, which is thatthe converter feed has a low NH₃ content, about 1.4%. However, thisadvantage is offset by other factors such as the heat required forregeneration of the molecular sieves, the operating complexity becauseof the requirement for numerous switching valves needed for the cyclicoperation to adsorb and desorb H₂O from the molecular sieves, highermaintenance costs and the high capital cost of the molecular sievevessels, heat exchangers, filters, piping and valves. The energy savingis estimated to be about 0.53 MM Btu/ST (where ST means short ton or2000 pounds), compared to a standard secondary flash design.

[0009] Another prior art concept is shown in U.S. Pat. No. 3,349,569.This patent discloses installation of an NH₃ scrubber at the inlet ofthe synthesis gas compressor, to use liquid NH₃ to absorb H₂O frommake-up synthesis gas. This allows make-up gas to be mixed with therecycle gas and to be fed directly to the ammonia converter. Theconverter effluent then goes directly to a cooling/chilling system ofthe type described above in connection with the use of molecular sieves.A substantial chilling effect takes place because of the heat requiredto vaporize NH₃, which comes from chilling the make-up gas. Theessentially H₂O-free make-up gas, which contains about 4.9% NH₃, is thenmixed with recycle gas as described above in connection with the use ofmolecular sieves.

[0010] There are several disadvantages with this system. Over-chillingof make-up gas due to excessive NH₃ evaporation resulting from lowpressure results in a scrubber overhead and compressor inlet temperature(−27° F.) which is below the minimum (−20° F.) for standard materials ofconstruction. More expensive low-temperature materials of constructionare needed for the scrubber, and the compressor will have to be re-rated(if possible). A re-rating of the compressor can sometimes be done ifits original materials of construction were satisfactory for more severeoperating conditions. Otherwise, an upgrade of the compressor lowpressure case may be required and this is costly. Another disadvantageof this method is that NH₃ will be contained in recycle gas sent to thefront end of the plant for desulfurization, thereby lowering plantefficiency. This NH₃ will be decomposed into H₂ and N₂ in the reformingsection setting up a recycle loop. The suction scrubber is also at adisadvantage from a moisture removal standpoint since the equilibriumH₂O content, although low, will be about two to three times higher thanwith the synthesis loop dehydrator of the present invention. The maindisadvantage, however, of this prior art system stems from its lowpressure operation and the resulting addition of a substantial quantityof NH₃ to the converter feed gas which contains about 2.6% NH₃. Thisreduces the energy savings potential to about 0.45 MM Btu/ST compared toa standard design with secondary flash (e.g. U.S. Pat. No. 1,815,243).

[0011] The version of the suction scrubber as described in U.S. Pat. No.3,349,569 can use further cooling and chilling between compressor stagesto condense some of the NH₃ that was vaporized in the compressor inletscrubber in the first place. The liquid NH₃ formed serves to furtherpurify the synthesis gas by random absorption of some of the remainingimpurities. However, the refrigeration requirements of such a systemwould be prohibitive.

[0012] Yet another prior art system places the scrubber at the samepressure as the synthesis loop, i.e., about 1900 psia, which leads toits one advantage: minimizing the NH₃ content in the scrubber overhead(2.7%) and in the converter feed (2.1%). There are, however, a number ofdisadvantages to this scheme. The most important one is the necessity tomodify the second-stage case of the compressor in the case of a revampof a 1900-2000 psia synthesis loop. A fourth nozzle must be added (achange that has never been done before) and the recycle wheel must bereduced in size. For the less common higher pressure loops (2500-3000psia), the compressor second case already has four nozzles so additionof a nozzle is not an issue here. The risk involved with this type ofmodification of the compressor is substantial, since a number ofproblems (vibration, surge, oil leakage, bearing failure, etc.) canresult. Further, the cost of the system for a retrofit is expected to bevery high because of the compressor modification, the required additionof two more heat exchangers (scrubber inlet coolers) and the need for anNH₃ pump. There is no compressor speed reduction as there is no NH₃evaporation and subsequent chilling for the make-up (first or secondstages). Energy savings for a system with 36° F. scrubber feed (avoidinga freezing problem) is expected to be about 0.44 MM Btu/ST.

SUMMARY OF THE INVENTION

[0013] Generally, the present invention provides a process and apparatusfor producing ammonia from a pressurized synthesis gas comprising amixture of hydrogen and nitrogen, which utilizes a dehydrator to removeH₂O from the synthesis gas at an intermediate stage of the synthesis gascompressor.

[0014] In a preferred embodiment, the present invention provides for theuse of substantially anhydrous liquid NH₃ for scrubbing and subsequentcooling in the dehydrator of synthesis gas withdrawn between the firstand second stages of a multi-stage compressor. This effects purificationof the make-up gas and also reduces compression power requirements.

[0015] The present invention further integrates the improvedpurification step in the synthesis loop in such a way as to enhance theefficiency of the processing steps. Scrubbing make-up synthesis gas withliquid NH₃ to remove impurities (mainly H₂O) allows the synthesis gas tobe mixed with recycle gas and fed directly to the converter. Morespecifically, purification of the make-up gas allows that gas to bemixed with NH₃-lean gas for feeding the third or recycle stage of thecompressor and then, the converter. Product NH₃ is not compressed in therecycle wheel, which saves power. Converter effluent can be sentdirectly to the cooling/chilling system for NH₃ condensation, therebyavoiding dilution with make-up gas and reducing refrigerationrequirements. Power expenditure is thus reduced as compared to prior artsystems. Product NH₃ is removed prior to recycle compression.

[0016] The present invention, as compared to prior art schemes, reducescompression power requirements and process energy requirements, allowsthe option for raising plant capacity, reduces compressor speeds,operates the purification step (removal of H₂O and other oxygenatedimpurities) at a pressure which is high enough to achieve sufficientpurification without having to resort to further processing steps, andeliminates the prohibitively expensive compressor interstagerefrigeration requirements required in some prior art schemes.

[0017] Specifically, in accordance with the present invention there isprovided an improvement in a process for the manufacture of ammonia. Theprocess comprises compressing in a multistage compressor a synthesis gascomprising hydrogen and nitrogen, each stage of the compressor having aninlet and a discharge associated therewith, contacting the compressedsynthesis gas in an ammonia reactor with a suitable catalyst underconditions to promote the reaction of a portion, less than all, of thehydrogen and nitrogen in the synthesis gas to ammonia, separatingproduct ammonia from a reactor effluent stream discharged from theammonia converter, and recycling a portion of the reactor effluentstream containing unreacted hydrogen and nitrogen to the multi-stagecompressor. The process includes withdrawing a make-up synthesis gasstream from the compressor and cooling and dehydrating the withdrawnsynthesis gas stream, the dehydrating step being carried out bycontacting the withdrawn synthesis gas stream with liquid ammonia, andreturning the cooled and dehydrated synthesis gas stream to thecompressor. The improvement comprises that the withdrawn synthesis gasstream is withdrawn from the discharge of the first stage of thecompressor and returned to the compressor at the inlet of the secondstage of the compressor.

[0018] Another aspect of the invention provides that the entiresynthesis gas stream is withdrawn from the discharge of the first stageof the compressor and cooled and dehydrated.

[0019] In a specific aspect of the invention, the multi-stage compressoris a three-stage compressor and the synthesis gas is discharged from thefirst stage at a pressure of from about 800 to 900 psia, is dischargedfrom the second stage of the compressor at a pressure of about 1800 to1900 psia, and is discharged from the third stage of the compressor at apressure of about 2000 to 2100 psia.

[0020] In one aspect of the invention, the withdrawn synthesis gasstream is cooled to a temperature of from about −20.5 to −26.1° C. (−5to −15° F.) prior to being returned to the compressor.

[0021] In another aspect of the present invention, the synthesis gasstream is returned to the compressor from the dehydrator without beingwarmed.

[0022] Another aspect of the invention provides that the H₂O content ofthe withdrawn synthesis gas stream is reduced to less than 0.1 parts permillion by volume prior to being returned to the compressor.

[0023] The invention also includes cooling the synthesis gas withdrawnfrom the compressor to condense ammonia contained therein and removingthe condensed ammonia from the synthesis gas prior to introducing itinto the ammonia converter.

[0024] The synthesis gas typically contains hydrogen and nitrogen in amolar ratio of about 3:1.

[0025] Yet another aspect of the invention provides an improvement in anapparatus for carrying out a process for the manufacture of ammonia bycompressing in a multi-stage compressor having at least a first stageand a second stage a synthesis gas comprising hydrogen and nitrogen,each stage of the compressor having an inlet and a discharge associatedtherewith. The process comprises contacting the compressed synthesis gasin an ammonia reactor by contacting the compressed synthesis gas with asuitable catalyst under conditions to promote the reaction of a portion,less than all, of the hydrogen and nitrogen in the synthesis gas toammonia and separating product ammonia from a reactor effluent streamdischarged from the ammonia converter. The process further comprisesrecycling a portion of the reactor effluent stream containing unreactedhydrogen and nitrogen to the multi-stage compressor, and contacting themake-up synthesis gas with liquid ammonia in a dehydrator having asynthesis gas inlet, a synthesis gas outlet and a liquid ammonia inletand a liquid ammonia outlet. The improvement to the apparatus comprisesthat the compressor is fitted with (a) a synthesis gas outlet connectingin flow communication the discharge of the first stage with thesynthesis gas inlet of the dehydrator, and (b) a synthesis gasintermediate inlet connecting the inlet of the second stage in flowcommunication with the synthesis gas outlet of the dehydrator, wherebyto define a synthesis gas flow path from the discharge of the firststage, through the dehydrator, thence to the inlet of the second stage.

[0026] The synthesis gas and liquid ammonia inlets and outlets arepreferably arranged to flow the liquid ammonia countercurrently to thesynthesis gas in the dehydrator.

[0027] An apparatus aspect of the present invention provides that theapparatus further comprises a heat exchanger to cool the synthesis gasand a liquid-vapor separator to separate H₂O therefrom, the heatexchanger and liquid-vapor separator being disposed in the synthesis gasflow path between the first stage of the compressor and the synthesisgas inlet of the dehydrator.

BRIEF DESCRIPTION OF THE DRAWING

[0028] The sole FIGURE is a schematic flow chart illustrating anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0029] In the manufacture of ammonia, synthesis loop make-up gas fromthe front section of the plant consists mainly of a mixture of hydrogen(H₂) and nitrogen (N₂) in approximately a 3:1 molecular ratio. The gasalso contains lesser amounts of inerts such as methane (CH₄) and argon(Ar) as well as undesirable trace impurities such as carbon monoxide(CO), carbon dioxide (CO₂) and water vapor (H₂O). In an ammoniasynthesis loop, it is imperative that oxygen-containing compoundsincluding H₂O be removed before the gas is introduced to the ammoniaconverter, as they are poisons to the synthesis catalyst. Such compoundstend to oxidize the catalyst, having a deleterious effect on it.

[0030] The present invention uses liquid NH₃ in a dehydrator to absorbH₂O and minor amounts of other impurities from make-up synthesis gas atan intermediate stage of compression of the gas. This allows make-up gasto be mixed with recycle gas and fed to the ammonia converter withconverter effluent then going directly to the cooling/chilling system.In accordance with the present invention, the dehydrator treats gastaken from an intermediate stage of the compressor. For mostapplications, e.g., 2000 psia synthesis loops with two make-upcompression stages, the scrubber treats gas at the inlet of the secondstage of the synthesis gas compressor and operates at medium pressure.For these designs, the dehydrator will operate at pressure of about800-900 psia. For less common higher pressure synthesis loops (2500-3000psia) with three make-up stages, the dehydrator will better be placedbetween the second and third stages of the compressor and operate at apressure of about 1200-1400 psia.

[0031] Just as there is an optimal operating pressure for the NH₃synthesis loop (about 1500-2500 psia depending on several factors),there is an optimal operating pressure for the H₂O removal operationinvolving contact with liquid NH₃. For standard synthesis loopsoperating at 1900-2000 psia the optimal H₂O removal operating pressurehas been found to be 800-900 psia, which is the operating point betweenthe two compressor cases. This pressure range has been found to be bestdue to the following factors:

[0032] Enhanced energy saving (about 0.50 MM Btu/ST for plants with anenergy requirement of 32 MM Btu/ST)

[0033] Enhanced compressor speed reduction (up to about 3% for thesynthesis gas compressor and about 4 to 5% for the refrigerationcompressor).

[0034] Enhanced potential capacity increase (3-4% if synthesis gascompressor turbines limit)

[0035] Reduced complexity (no recycle of NH₃ to front end)

[0036] Reduced capital investment (no special expensive materials ofconstruction are required, no modifications to the synthesis gascompressor are needed, and no additional exchangers such as the scrubberinlet coolers for the high pressure unit are required, no extrainterstage refrigeration is needed).

[0037] The discussion below pertains to a standard NH₃ synthesis loopusing a three-stage synthesis gas compressor (two make-up stages and onerecycle stage). For this synthesis loop, the dehydrator is, inaccordance with the present invention, located between the first twocompression stages. This location has been found to be the optimalposition (optimal operating pressure) for a number of reasons previouslygiven.

[0038] Referring to the sole FIGURE which schematically illustrates thedehydrator employed in a nominal 2000 psia ammonia synthesis loop, themake-up synthesis gas stream 1, derived from well-known prior processsteps (such as steam reforming of hydrocarbon feed followed by shiftconversion, CO₂ removal and methanation) enters at a pressure of about300-400 psia. There can be some variation of this pressure depending onthe upstream design but this has no bearing on the present invention.The gas consists mainly of reactants hydrogen (H₂) and nitrogen (N₂) inan approximate molar ratio of 3:1. Other components such as methane(CH₄) and argon (Ar) are usually present in small amounts (about 1%total). Oxygen containing impurities such as carbon monoxide (CO),carbon dioxide (CO₂) and water vapor (H₂O) are also present. The carbonoxides have already been virtually eliminated from gas stream 1 by theupstream methanator, but H₂O must still be removed prior to introducingthe synthesis gas to the loop and allowing it to enter the NH₃ converter60.

[0039] Gas stream 1 is compressed to about 800-900 psia in the firststage 50 of a synthesis gas centrifugal compressor. The discharge stream2 from the first stage is split, with a portion thereof, stream 3, beingrouted to the front end of the plant for use as hydrodesulfurizationgas, as is known in the art. The bulk of gas stream 2 is flowed viastream 4 to heat exchanger 52 (which may consist of several differentunits) and cooled therein to a temperature of about 4.4° C. (40° F.).Most of the H₂O present is condensed and separated in drum 53 and exitsthe system as stream 7.

[0040] Stream 6, containing about 160 ppm H₂O, is the vapor streamleaving the drum 53 and flowing to dehydrator 54, where it is scrubbedwith essentially anhydrous NH₃ contained in stream 26. Dehydrator 54 canbe one of any number of known gas-liquid contacting devices that bringgas and liquid phases into intimate contact with each other for thepurpose of a diffusional exchange. Water in the gas phase is absorbed byammonia in the liquid phase within dehydrator 54, which typically may bea tower using bubble cap trays, sieve trays, packing, or any suitableknown means to effectuate intimate vapor-liquid contact. For thisapplication, bubble cap trays are preferred for insuring adequatevapor-liquid contact because a level of liquid is maintained on eachtray. The gas is contacted countercurrently with liquid NH₃ for removalof most of the impurities and essentially all of the H₂O. In the towerof dehydrator 54, gas flows upwardly and contacts liquid flowingdownwardly. In absorption, the component being absorbed is depleted inthe gas phase as it moves up the column and increased in the liquidphase as it flows down.

[0041] The final water content in the exit gas will be that inequilibrium with liquid leaving the stage (nearly pure NH₃ with a verysmall amount of H₂O). The water content in the exit gas must be below 10ppm such that the converter feed gas, after dilution with recycle gas,will contain no more than 1-2 ppm H₂O. In actual practice, it isexpected that the water content will be much lower and virtuallynon-detectable. By calculation, the H₂O concentration in the vapor isreduced to less than 0.1 ppm after the first theoretical tray, and toessentially zero after the second theoretical tray. Although the H₂Ocontent is expected to be this low, the effectiveness of the dehydratorwill not be materially compromised even if the H₂O content of theoverhead is somewhat higher (up to about 5 ppm). Experimental datareported in U.S. Pat. No. 3,349,569 concerning water/ammonia equilibriaindicates that the H₂O content in overhead stream 8 leaving dehydrator54 would be satisfactorily low (in the range of 1 ppm after correctingfor inlet concentration and operating pressure). A substantial coolingeffect takes place to provide the heat for vaporization of NH₃ thatsaturates the gas. The dehydrated scrubber overhead leaves as stream 8at a temperature of about −10° F. containing about 3.5% NH₃. A liquidlevel is maintained in the bottom of the tower comprising dehydrator 54and net liquid leaves as stream 27.

[0042] The overhead exit stream 8 is compressed to about 1900 psia inthe second stage 56 of the compressor. Discharge stream 9 from secondstage 56 is then mixed with recycle gas from stream 31 to form stream10, which is further compressed to about 2030-2080 psia in the thirdstage 57 of the compressor. The third stage 57 is sometimes referred toas the “recycle wheel”. The exact discharge pressure will depend on thesynthesis loop pressure drop, which is a function of specific loopdesign, capacity, NH₃ conversion and other factors. The combined makeupand recycle gas stream 11 then exits the third stage 57 of thecompressor and is preheated in feed/effluent heat exchanger 59. Thepreheated gas then flows as stream 12, containing about 2.3% NH₃, toammonia converter 60. Here the NH₃ synthesis reaction takes place over acatalyst, the reaction being shown by the following equation.

3H ₂ +N ₂=2NH ₃

[0043] Converter exit gas stream 13, containing from about 12 to 20%NH₃, usually about 15 to 17% NH₃, then flows through heat recovery heatexchanger 61. Gas leaves this heat exchanger 61 as stream 14 and isfurther cooled in heat exchanger 59, and leaves it as stream 15. Furthercooling of the gas stream 15 is effected with cooling water in heatexchanger 62. Exit gas emerges from water-cooled exchanger 62 as stream18 which is split as shown into streams 16 and 17 which enter,respectively, heat exchangers 64 and 66. Additional cooling with asuitable refrigerant, such as NH₃, is carried out in heat exchanger 64,which may consist of several units using progressively colder levels ofrefrigeration. Refrigeration recovery is carried out in heat exchanger66. Respective exit streams 19 and 20 are then combined into stream 21which flows to product separator 67. Here, NH₃ product is removed as theliquid phase, stream 23. The vapor phase, stream 22, returns to heatexchanger 66 for refrigeration recovery as noted above. Rewarmed vapor,stream 30, is split with the smaller stream 32 being purged to fuel toremove inerts. Most of the stream is returned to the compressor asrecycle stream 31. It will be noted that a purge gas (line 32) chillerand separator are not required in the illustrated arrangement.

[0044] The liquid stream from product separator 67, stream 23, is splitwith a portion being routed to the dehydrator 54 as stream 25. Thepressure of stream 25 is reduced across flow control valve 55, with exitstream 26 from valve 55 flowing to the top of the dehydrator. The restof the high pressure liquid from product separator 67 goes to letdowndrum 69 as stream 24. This vessel operates at reduced pressure, about250-270 psia. Dehydrator bottoms liquid NH₃ stream 27, containing H₂Oand minor amounts of other impurities removed from the make-up synthesisgas, is taken out through level control valve 71 and sent to drum 69 viastream 33. Flash gas, stream 28, leaves letdown drum 69 overhead as fuelwhile liquid NH₃ product, stream 29, is removed from the bottom ofletdown drum 69.

[0045] Referring to the FIGURE, the temperature of the gas exiting thedehydrator 54 via overhead stream 8, and its NH₃ concentration, willvary somewhat as they are a function of the feed gas temperature (stream6), the liquid NH₃ temperature (streams 25 and 26) and the operatingpressure. In general, it is better to minimize the operating temperature(down to about −20° F. for stream 8) as this lowers the NH₃ vaporpressure and reduces the quantity of NH₃ in the make-up gas andultimately, the concentration of NH₃ in the converter feed gas fed bystream 12 to ammonia converter 60. Minimum energy requirement occurswhen the NH₃ concentration rise across the converter is maximized. Also,a lower compressor inlet temperature reduces the inlet volumetric flow,power requirement and speed as discussed elsewhere. In case thetemperature from exchanger 52 is relatively high (e.g., when it is not arefrigerated chiller), it will be prudent to provide further cooling tolower the temperature of stream 6 to about 40° F.

[0046] There can also be some variability with regard to the amount ofscrubbing liquid used in stream 25 sent to the top of dehydrator 54. Ithas to be at least equal to the amount of NH₃ vaporized to avoidevaporation to dryness in the dehydrator. In practice, a certain marginwill be added to the calculated minimum so the quantity should be atleast 10% of stream 23 leaving the separator 67. When the temperature ofstream 25 is very close to dehydrator 54 top temperature (e.g., −10°F.), the quantity of scrubbing liquid supplied by stream 25/26 haslittle effect on the dehydrator heat balance, so its flow should be inthe 10 to 15% range of stream 23. When stream 25 temperature is warmer(e.g., −2° F.), its flow should be reduced to about 10-15% of stream 23,as greater amounts cause a slight warming trend with a little more NH₃going overhead. When stream 25 temperature is colder (e.g., −18° F.),its flow can be increased to at least 15 to 20% of stream 23, sinceincreased amounts of flow give a cooling trend reducing the NH₃concentration in the dehydrator overhead.

[0047] The dehydrator 54 may be a column using a small number of trays(preferably bubble cap) with a sump in the bottom containing liquid NH₃maintained under level control. A kick-back cooler (not shown in theFigure) for the synthesis gas compressor may be included to handleoperation under recycle conditions (during startup). Installation of aseparator (not shown in the FIGURE) at the discharge of the compressormay be necessary to remove oil in the unlikely event of oil carryoverfrom the compressor. An NH₃ pump is not needed to supply the liquidammonia to dehydrator 54 for the scrubbing step. This is because thedehydrator 54 is operated at medium pressure, well below the pressure ofseparator 67 which supplies the liquid NH₃. This is a significantimprovement over the prior art high pressure scrubber variation thatrequires a pump plus a spare. For example, in a nominal 2000 psiasynthesis loop, separator 67 will be at a pressure of about 1950 psiawhile dehydrator 54 is at a pressure of about 800-900 psia. It should benoted that the scrubbing is confined to the make-up stream alone, and isnot required for the combined make-up/recycle stream as required in, forexample, U.S. Pat. No. 1,830,167 and Canadian Patent 257,043.

[0048] Some synthesis loop piping modifications will be necessary for aretrofit of the dehydrator of the present invention into an existingplant, but not, of course, for a new plant. The compressor recycle wheeldischarge is connected to the tubeside inlet of heat exchanger 59.Converter effluent from the shellside of exchanger 59 is routed toexchanger 62 inlet. Flash gas from separator 67, after flowing throughthe tubeside of exchanger 66 and after purge withdrawal, is then routedto the recycle wheel inlet. Small liquid NH₃ lines from separator 67 tothe dehydrator 54 and from the bottom of the dehydrator to drum 69 arerequired.

[0049] The present invention has one or more of the followingcharacteristics and advantages over prior art processing schemes.

[0050] Synthesis make-up gas purification is thus attained in a singlestep (in the dehydrator), as opposed to using multiple steps as in theprior art. In contrast, U.S. Pat. No. 3,349,569 shows that NH₃ iscondensed after the suction scrubber between stages of the compressor towash and further purify the gas. The present invention reduces capitaland operating costs by avoidance of added compressor interstagerefrigeration requirements, as required, for example, in the scheme ofU.S. Pat. No. 3,349,569. Further, undesirable recycle of NH₃ to thefront end of the plant, such as occurs with the suction scrubber of U.S.Pat. No. 3,349,569, is avoided.

[0051] A reduction in the speed of the synthesis gas compressor isobtained by diverting to the dehydrator gas taken from an intermediatestage of the compressor; more specifically, by diverting the dehydratorgas from between the first and second stages of the compressor. Forexample, with reference to the FIGURE, there is diverted as dehydratorgas the gas exiting from first stage 50 of the compressor. The divertedgas is, as described above, cooled in heat exchanger 52 and dehydratedin dehydrator 54 before being flowed via stream 8 to the second stage 56of the compressor, at a temperature of about −23.3° C. (−10° F.). Thisresults in load reduction for both the second stage and the third stage(“recycle wheel”) of the compressor, as described in more detail below.This benefit is not obtained with prior art processing schemeconfigurations. The second stage inlet temperature is cooler,approximately −10° F., at the exit from the dehydrator, as compared toabout 40-45° F. for a standard prior art design. The cooler temperaturereduces the compression load for the second stage. Also, the secondstage discharge temperature is correspondingly lower, as it isdetermined from the equation T₂=T₁*(P₂/P₁ ^(n-1/n)−1) wherein T₁=inlettemperature, T₂=outlet temperature, P₁=inlet pressure, P₂=outletpressure and (n−1)/n=(k−1)/(k*ep) where k=Cp/Cv and ep=polytropicefficiency of the compressor. Cp=specific heat at constant pressurewhile Cv=specific heat at constant volume.

[0052] A lower inlet temperature results in a lower dischargetemperature. This means that the mixed inlet temperature to the recyclewheel is lower than the standard prior art design, since there is nocooling of the second stage discharge before mixing with recycle.Integration of the dehydrator into the process can increase theproduction capacity by about 3-4% if the synthesis gas compressor drivepower output limits production, and by about 8-9% if the refrigerationcompressor drive power output limits production. Integration of thedehydrator into the process provides mild operating conditions relativeto prior art schemes, which means that no special low temperaturematerials of construction (those designed for temperatures of less than−20° F.) are required.

[0053] Integration of the dehydrator into the process reduces synthesisloop pressure drop which is about 5% lower than the suction scrubber ofU.S. Pat. No. 3,349,569. In the system of the present invention, the NH₃content of the dehydrator overhead discharge is 3.5% (vs. 4.9% for thesuction scrubber) and the NH₃ content of the converter feed is lower,e.g., 2.3% versus 2.6% for the suction scrubber. This results in reducedcirculation for a given capacity and, therefore, lower pressure drop.

[0054] Safe and continuous operation of the converter is assured sincethe moisture removal will be at least as complete (if not more so) thanit is with prior art designs. Water removal will be accomplished in adehydrator comprising a specially designed tower dedicated to thatpurpose rather than as it is now by random contact with liquid NH₃ asthe compressor discharge stream flows through the chillers and piping tothe separator. The latter approach is illustrated by U.S. Pat. No.1,815,243.

[0055] The dehydrator of the present invention is connected to treatonly the make-up gas instead of the combined make-up and recycle streamas shown, for example, in U.S. Pat. No. 1,830,167 and Canadian Patent257,043. This greatly reduces its size and cost.

[0056] The dehydrator operation is continuous and therefore much simplerthan with prior art systems using molecular sieves. There are noexpensive and high-maintenance switching valves required in the systemof the present invention as is the case with a molecular sieve processscheme. Further, the installed cost of the system of the presentinvention is much (60-70%) less than a molecular sieve system. Thedehydrator energy saving is comparable to that achieved with a molecularsieve system.

[0057] The dehydrator water-removal scrubbing step is, as noted above,advantageously located between the first two stages of the synthesis gascompressor. Intermediate stage dehydration is superior for a number ofreasons including reduced energy requirements, reduced synthesis gascompressor speed requirements and increased production.

[0058] The elevated pressure at which the dehydrator of the presentinvention operates (e.g., about 800-900 psia) is satisfactory to achieveadequate and essentially complete H₂O removal without resorting tofurther contact with liquid NH₃. Use of the dehydrator to scrub make-upsynthesis gas with liquid NH₃ at elevated pressure removes impurities(mainly H₂O but also trace amounts of CO and CO₂) to render the make-upsynthesis gas suitable for catalytic NH₃ synthesis. The H₂O removal isessentially complete with only 0.1 ppm remaining after only onetheoretical stage of liquid-gas contact in the dehydrator, leading tosatisfactory converter performance and long catalyst life. Standardammonia converter catalyst vendors specify a maximum atomic oxygencontent of 3 ppm in the feed stream. The most common synthesis loopsystems employ a secondary flash (see U.S. Pat. No. 1,815,243 and3,350,170) and rely on the contact between condensing NH₃ and synthesisgas in the exchangers and piping to accomplish H₂O removal. It has beenfound that the H₂O removal with that prior art method is far fromcomplete, with some measurements indicating a level of 15 ppm H₂O in theconverter feed. While this is marginally satisfactory for standardammonia synthesis catalysts (although it contributes to shorter catalystlife), it is not acceptable for recently developed precious metalammonia synthesis catalysts.

[0059] In accordance with the present invention, the synthesis loop isreconfigured for optimal operation and maximum energy saving. As aresult of use of the dehydrator to dry the interstage synthesis gas, H₂Ohas been removed so reconfiguration of the synthesis loop is possible.The scrubbed make-up gas can be mixed with NH₃ lean recycle gas and feddirectly to the ammonia converter (60 in the FIGURE). Ammonia convertereffluent can be then sent directly to the cooling/chilling steps(avoiding dilution with make-up gas) and saving refrigeration power.Recycle gas after NH₃ removal (in separator 67 of the FIGURE) and afterpurge withdrawal (via line 32 of the FIGURE) is then routed to the third(recycle) stage of the compressor. Product NH₃ is not compressed in therecycle stage, which saves power. Also, reconfiguration of the synthesisloop results in a lower synthesis gas compressor discharge pressure fora fixed ammonia converter outlet pressure. This occurs because there arefewer pieces of equipment between the compressor and the converter thanwith prior art schemes. This is one of the factors which reduces powerand contributes to a lower energy consumption. A major reason forreduced energy consumption in the reconfigured loop is that the extraNH₃ introduced via the dehydrator can be condensed in the loop withcooling water rather than by refrigerated chillers because of theelevated converter effluent dew point. Thus, a large gain is made in thesynthesis gas compressor power without sacrificing power in therefrigeration compressor. Such reconfiguration of the ammonia synthesisloop also allows greater heat recovery in the heat exchanger (item 61 inthe FIGURE) used to heat boiler feed water by heat exchange with theeffluent of the ammonia converter (item 60 in the FIGURE) since energyinput into the third stage, or recycle wheel (item 57 of the FIGURE) ofthe compressor is directed to the ammonia converter. In contrast, priorart designs such energy is instead directed to a cooling water exchangeras in secondary flash designs such as those of U.S. Pat. Nos. 1,815,243and 3,350,170. A further gain realized by reconfiguring the synthesisloop is elimination of the purge gas chiller and its separator. Thissaves capital in a new design and saves energy whether the design is newor a retrofit. The reason for this is that the purge gas is chilled bysuccessively colder levels of refrigeration in the synthesis loopreconfigured in accordance with the practices of the present invention,while it is chilled by only the coldest level in the prior art designs.Overall, a significant total energy saving of about 0.5 MM Btu/ST ofproduct results from utilization of the interstage dehydrator of thepresent invention (“ST”=short ton or 2000 pounds of product NH₃).

[0060] Even where the original synthesis loop configuration is retained,the dehydrator (54 in the FIGURE) positioned between the first (50 inthe FIGURE) and second (56 in the FIGURE) compressor stages may be usedto advantage for the removal of water (and other impurities) from thesynthesis gas. Without the benefit of appreciable energy saving attainedby reconfiguring the synthesis loop as described above, the energy savedin the synthesis gas compressor is mostly offset by higher refrigerationpower requirements. The main gain in this case will be a significantimprovement in the converter catalyst life due to lower H₂O content.Also, load will be transferred from the synthesis gas compressor to therefrigeration compressor. A speed reduction of about 2% will be realizedfor the synthesis gas compressor. This can be advantageous in plantswhere the synthesis gas compressor is limiting and there is extracapacity available in the refrigeration compressor (e.g., in coldclimates or winter in warm climates). More plant capacity can beobtained under these conditions.

[0061] In accordance with the practices of the present invention, thescrubbing step is located between the first two stages (50 and 56 in theFIGURE) of the synthesis gas compressor. This position is superior for anumber of reasons including reducing the energy requirements, reducingthe synthesis gas compressor speed requirement and increasing productproduction. Mild operating conditions mean that no special lowtemperature materials are required, and in revamping of existing plantsthe existing metallurgy can be retained. In new plants, low-temperaturemetallurgy requirements are reduced. Withdrawal and dehydrating andcooling between the first and second stages avoids undesirable recycleof NH₃ to the front end of the plant that occurs when a suction scrubberis used, as in Nebgen U.S. Pat. No. 3,349,569).

[0062] The elevated pressure of the dehydrator (about 800-900 psia) issatisfactory to achieve adequate and essentially complete H₂O removalwithout resorting to further contact with liquid NH₃ as in U.S. Pat. No.3,349,569.

[0063] Extensive interstage refrigeration requirements are avoided bythe practices of the present invention, as there is no need to cool andextensively chill the compressor interstage gas to condense NH₃. Incontrast, the refrigeration power expended to achieve the deep coolingof −5 to −50° C. (23 to −58° F.) mentioned in U.S. Pat. No. 3,349,569would equal or exceed the power saved for the synthesis gas compressorin that scheme. Also, the extensive investment required by the prior artdesign is avoided. For a retrofit (revamp of an existing plant), theexisting heat exchangers upstream of the dehydrator can be used.

[0064] The interstage cooling of synthesis gas between the first andsecond stages beneficially affects two stages of the synthesis gascompressor instead of just one. Equipment modification is readily madeto the most commonly employed compressor design which incorporates thesecond stage and the third stage (recycle) compressor in the same case,with the recycle wheel compressing the combined make-up and recyclesynthesis gas stream. The chilled gas from the dehydrator enters thesecond stage of the compressor and, after compression, the dischargetemperature is cooler since it is set by the relationship T₂=T₁^((n-1)/n). This stream, without further cooling, is then mixed withrecycle so the combined stream temperature is lower. Lower inlettemperatures to the second and third stages result in lower powerrequirements for both stages since they are set by the well-knownpolytropic relationship P=K/ep*MPH*T₁*Z*n/(n−1)) *P₂/P₁ ^((n-1)/n)−1)wherein P=power, K is a constant, MPH=molar flow in moles per hour,Z=compressibility, and ep, n, P₁, P₂ and T₁ are as defined above.Cooling of the second stage inlet more than offsets the increased molarflow as shown below, where a 7% power saving is obtained for that stagealone. Other factors in the above equation are essentially constant,giving

P=(460−10)/(460+41)*(1.035/1.000)=0.93

[0065] For the recycle stage, the inlet temperature is lower for thereason mentioned above. Also, the inlet temperature and flow (since theproduct NH₃ is not compressed) are advantageously influenced by the loopreconfiguration, so a 12% power saving is realized, as shown by thefollowing calculation.

P=(460+114)/(460+150)*(0.935/1.000)=0.88

[0066] In addition, the recycle power is reduced somewhat further owingto lower pressure drop. The first-stage power will be about the same, sothe overall power reduction for the synthesis gas compressor is about6%. This is shown simplistically by

P=(1.0*0.36)+(0.93*0.36)+(0.88*0.28)=0.941

[0067] Of course, there is some redistribution of power between stagesto satisfy the compressor performance curves when the dehydrator isemployed. This results in a lower pressure ratio for the first stage anda correspondingly higher pressure ratio for the second stage. Still, theoverall power saving remains approximately 6%. The inlet volumetricflows are also less, resulting in lower compressor speed (about 3% witha reconfigured synthesis loop) and consequently, lower operatingseverity. This advantage (dehydrator positively affecting two stages) isnot realized with the prior art designs.

[0068] Liquid NH₃ is supplied to the scrubbing step from a downstreamseparator at higher pressure. A pump is not required during normaloperation.

[0069] Scrubbing is confined to the make-up stream alone, not thecombined make-up plus recycle stream as presented in U.S. Pat. Nos.1,830,167 and 257,043. (It is noted that in U.S. Pat. No. 3,349,569 onlythe make-up stream is scrubbed.)

[0070] Synthesis loop pressure drop is lower than with a suctionscrubber, e.g., as shown in U.S. Pat. No. 3,349,569, assuming that thereis no further NH₃ condensation between stages. This is because of thelower NH₃ concentration in the overhead of the dehydrator (54) ascompared to that in the prior art suction scrubber (3.5% vs. 4.9%), andbecause of higher pressure (850 vs. 350 psia) in the dehydrator ascompared to that of the prior art suction scrubber. The NH₃concentration in the feed to the ammonia converter (60) is thereforelower (2.3% vs. 2.6%) than in the prior art scheme. With a fixed NH₃concentration in the converter outlet, there will be a larger change inconcentration across the converter in the scheme of the presentinvention as compared to that of U.S. Pat. No. 3,349,569, whichtranslates into a lower circulation for a given capacity and lowerpressure drop. This also results in a lower recycle power requirement.

[0071] The amount of liquid NH₃ used for scrubbing should normally bebetween 10-15% of the total liquid from the high pressure separator. Byreducing the quantity of liquid ammonia used, the reintroduction ofinerts to the synthesis loop is reduced, as is the size and cost of theliquid piping, valves and scrubbing equipment.

[0072] The make-up gas is precooled (usually to about 38-45° F. in arefrigerated chiller) prior to scrubbing. The precooling is followed bya knockout drum to remove condensed H₂O. This reduces the H₂O content ofthe saturated gas and the load on the scrubber. Precooling also lowersthe NH₃ content of the dehydrator overhead vapor and consequently, theNH₃ content of the converter feed. Further, it reduces the overheadtemperature with the favorable effect on compressor power and speedalready alluded to.

[0073] A less common design of ammonia synthesis plant uses a 2500-3000psia synthesis loop that employs a four-stage compressor (three make-upstages and one recycle stage). Usually, two make-up stages are containedin the first case of the compressor and the second case of thecompressor is of a four-nozzle design. Here, a dehydrator such asdehydrator 54 could be located after the first make-up stage, after thesecond make-up stage, or at synthesis loop pressure after the thirdmake-up stage. In short, the make-up synthesis gas may be taken from thecompressor to the dehydrator from any intermediate compression stage andreturned to the compressor at the inlet to the next stage.

[0074] Preferably, however, in such case the gas should be taken afterthe second make-up stage for three reasons. At that stage ofcompression, the pressure would be high enough to insure adequate H₂Oremoval. At the same time, the chilling effect would benefit the thirdmakeup compression stage, thereby lowering its power requirement andspeed. Finally, in revamping existing plants, a chiller is alreadypresent at this location.

[0075] For such higher pressure synthesis loops (2500-3000 psia), thedehydrator will better be placed between the second and third stages ofthe compressor and operate at a pressure of about 1200-1400 psia. Thus,in such four-stage compressor configurations, the synthesis gas isdischarged from the second stage of the compressor at about 1200 to 1400psia, and is discharged from the fourth stage of the compressor at apressure of about 2500 to 3000 psia.

[0076] Those skilled in the art will appreciate that numerous variationsmay be made to the specific embodiments described above, whichvariations nonetheless lie within the scope of the present invention asdefined in the appended claims.

What is claimed is:
 1. In a process for the manufacture of ammonia bycompressing in a multi-stage compressor a synthesis gas comprisinghydrogen and nitrogen, each stage of the compressor having an inlet anda discharge associated therewith, contacting the compressed synthesisgas in an ammonia reactor with a suitable catalyst under conditions topromote the reaction of a portion, less than all, of the hydrogen andnitrogen in the synthesis gas to ammonia, separating product ammoniafrom a reactor effluent stream discharged from the ammonia converter,and recycling a portion of the reactor effluent stream containingunreacted hydrogen and nitrogen to the multi-stage compressor,withdrawing a make-up synthesis gas stream from the compressor, coolingand dehydrating the withdrawn synthesis gas stream, the dehydrating stepbeing carried out by contacting the withdrawn synthesis gas stream withliquid ammonia, and returning the cooled and dehydrated synthesis gasstream to the compressor, the improvement comprising that the withdrawnsynthesis gas stream is withdrawn from the discharge of the first stageof the compressor and returned to the compressor at the inlet of thesecond stage of the compressor.
 2. The process of claim 1 wherein theentire synthesis gas stream is withdrawn from the discharge of the firststage of the compressor and cooled and dehydrated.
 3. The process ofclaim 1 or claim 2 wherein the multi-stage compressor is a three-stagecompressor and the make-up synthesis gas is discharged from the firststage at a pressure of from about 800 to 900 psia, is discharged fromthe second stage at a pressure of about 1800 to 1900 psia, and isdischarged from the third stage of the compressor at a pressure of about2000 to 2100 psia.
 4. The process of claim 3 wherein no cooling isimposed on the make-up synthesis gas between the second and third stagesof the compressor.
 5. The process of claim 3 wherein the synthesis gasstream is cooled to a temperature of from about −20.5 to −26.1° C. (−5to −15° F.) prior to being returned to the compressor.
 6. The process ofclaim 1 or claim 2 wherein the synthesis gas stream is returned to thecompressor from the dehydrator without being rewarmed.
 7. The process ofclaim 3 wherein the H₂O content of the withdrawn synthesis gas stream isreduced to less than 0.1 parts per million by volume prior to beingreturned to the compressor.
 8. The process of claim 1 or claim 2 whereinthe synthesis gas contains hydrogen and nitrogen in a molar ratio ofabout 3:1.
 9. The process of claim 1 or claim 2 further comprisingcooling the synthesis gas withdrawn from the compressor to condenseammonia contained therein and removing the condensed ammonia from thesynthesis gas prior to introducing it into the ammonia converter.
 10. Anapparatus for carrying out a process for the manufacture of ammonia bycompressing in a multi-stage compressor having at least a first stageand a second stage a synthesis gas comprising hydrogen and nitrogen,each stage of the compressor having an inlet and a discharge associatedtherewith, the process comprising contacting the compressed synthesisgas in an ammonia reactor by contacting the compressed synthesis gaswith a suitable catalyst under conditions to promote the reaction of aportion, less than all, of the hydrogen and nitrogen in the synthesisgas to ammonia, separating product ammonia from a reactor effluentstream discharged from the ammonia converter, and recycling a portion ofthe reactor effluent stream containing unreacted hydrogen and nitrogento the multi-stage compressor, and contacting the synthesis gas withliquid ammonia in a dehydrator having a make-up synthesis gas inlet, asynthesis gas outlet and a liquid ammonia inlet and a liquid ammoniaoutlet, the improvement comprising that the compressor is fitted with asynthesis gas outlet connecting in flow communication the discharge ofthe first stage with the synthesis gas inlet of the dehydrator, asynthesis gas intermediate inlet connecting the inlet of the secondstage in flow communication with the synthesis gas outlet of thedehydrator, whereby to define a synthesis gas flow path from thedischarge of the first stage, through the dehydrator, thence to theinlet of the second stage.
 11. The apparatus of claim 10 wherein thesynthesis gas and liquid ammonia inlets and outlets are arranged to flowthe liquid ammonia countercurrently to the synthesis gas in thedehydrator.
 12. The apparatus of claim 10 or claim 11 further comprisinga heat exchanger to cool the synthesis gas and a liquid-vapor separatorto separate H₂O therefrom disposed in the synthesis gas flow pathbetween the first stage of the compressor and the synthesis gas inlet ofthe dehydrator.